Firefox Nightly 74 just removed an important function (getFile) that the extension depends on.
The extension has been updated to workaround this issue, but the performance of saving files will decrease.
Additionally, it is unknown whether Firefox will remove the whole mutableFile API in the near future. If so, the entire storage layer will need to be rewritten. It may or may not be possible to build this extension with acceptable performance then.
I'll see how things go and prepare workarounds if possible.